Comparison operators (==, <, >, >=, <=) in Firebase - Mini Project: Build & Apply

Using Comparison Operators in Firebase Security Rules

📖 Scenario: You are managing a Firebase Firestore database for a simple blog app. You want to control who can read and write posts based on certain conditions.

🎯 Goal: Write Firebase security rules that use comparison operators ==, &lt;, &gt;, &gt;=, and &lt;= to allow or deny access to posts.

📋 What You'll Learn

Create a rule that allows reading posts only if the post's status is exactly "published".

Add a rule that allows writing a post only if the likes count is less than 100.

Add a rule that allows updating a post only if the views count is greater than or equal to 10.

Add a rule that denies deleting a post if the comments count is greater than 0.

Add a rule that allows reading posts only if the createdAt timestamp is less than or equal to the current time.

💡 Why This Matters

🌍 Real World

Firebase security rules protect your database by controlling who can read or write data based on conditions.

💼 Career

Understanding and writing secure Firebase rules is essential for backend and cloud developers working with Firebase.

1

Create the base Firestore rules structure

Write the initial Firestore rules with a match block for posts/{postId} and allow read only if resource.data.status == "published".

Firebase

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    match /posts/{postId} {
      // Your code here
    }
  }
}

Hint

Use resource.data.status == "published" to check the post status.

2

Add write rule with likes count check

Add an allow write rule inside match /posts/{postId} that permits writing only if request.resource.data.likes < 100.

Firebase

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    match /posts/{postId} {
      allow read: if resource.data.status == "published";
      // Your code here
    }
  }
}

Hint

Use request.resource.data.likes < 100 to limit writes.

3

Add update rule with views count check

Add an allow update rule inside match /posts/{postId} that permits updates only if request.resource.data.views >= 10.

Firebase

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    match /posts/{postId} {
      allow read: if resource.data.status == "published";
      allow write: if request.resource.data.likes < 100;
      // Your code here
    }
  }
}

Hint

Use request.resource.data.views >= 10 to allow updates.

4

Add delete and read rules with comments and createdAt checks

Add an allow delete rule that denies deletion if resource.data.comments > 0. Also, update the allow read rule to allow reading only if resource.data.createdAt <= request.time and resource.data.status == "published".

Firebase

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    match /posts/{postId} {
      allow read: if resource.data.status == "published";
      allow write: if request.resource.data.likes = 10;
      // Your code here
    }
  }
}

Hint

Use !(resource.data.comments > 0) to deny deletes if comments exist.
Use resource.data.createdAt <= request.time to check creation time.

Practice

1. Which comparison operator in Firebase queries checks if a field value is exactly equal to a given value?

easy

A. ==

B. <

C. >=

D. <=

Solution

1. Step 1: Understand the purpose of each operator

   The operator == checks for exact equality, while <, >=, and <= check for less than, greater or equal, and less or equal respectively.

2. Step 2: Identify the operator for exact equality

   Since the question asks for exact equality, the correct operator is ==.

3. Final Answer:

   == -> Option A

4. Quick Check:

   Exact equality operator = == [OK]

Hint: Use == to check if values are exactly the same [OK]

Common Mistakes:

• Confusing == with assignment operator =
• Using < or > for equality checks
• Thinking >= means equal only

2. Which of the following is the correct Firebase query syntax to find documents where the field age is greater than 18?

easy

A. query.where('age', '==', 18)

B. query.where('age', '<', 18)

C. query.where('age', '>=', 18)

D. query.where('age', '>', 18)

Solution

1. Step 1: Identify the operator for 'greater than'

   The operator for 'greater than' is >.

2. Step 2: Match the operator with the correct syntax

   The correct syntax uses query.where('age', '>', 18) to filter documents where age is greater than 18.

3. Final Answer:

   query.where('age', '>', 18) -> Option D

4. Quick Check:

   Greater than operator = > [OK]

Hint: Use '>' for values strictly greater than target [OK]

Common Mistakes:

• Using '==' instead of '>'
• Confusing '>=' with '>'
• Using '<' when wanting greater values

3. Given this Firebase query: query.where('score', '>=', 50).where('score', '<', 70), which scores will be included in the results?

medium

A. Scores greater than or equal to 50 and less than 70

B. Scores greater than 50 and less than or equal to 70

C. Scores equal to 50 or equal to 70

D. Scores equal to 50 and less than 70

Solution

1. Step 1: Analyze the first condition

   The first condition score >= 50 includes scores 50 and above.

2. Step 2: Analyze the second condition

   The second condition score < 70 includes scores less than 70, excluding 70 itself.

3. Step 3: Combine both conditions

   Only scores that satisfy both conditions are included: scores greater than or equal to 50 and less than 70.

4. Final Answer:

   Scores greater than or equal to 50 and less than 70 -> Option A

5. Quick Check:

   Range includes 50 ≤ score < 70 [OK]

Hint: Combine >= and < for range queries [OK]

Common Mistakes:

• Including 70 when using '<' operator
• Confusing '>' with '>='
• Thinking 'or' instead of 'and' between conditions

4. You wrote this Firebase query: query.where('price', '=>', 100). Why does it cause an error?

medium

A. Because 'price' field cannot be compared

B. Because 100 should be a string, not a number

C. Because '=>' is not a valid comparison operator in Firebase queries

D. Because the operator should be '<=' instead

Solution

1. Step 1: Check the operator syntax

   The operator '=>' is not a valid Firebase comparison operator. The correct operator for 'greater than or equal' is '>='.

2. Step 2: Understand Firebase operator rules

   Firebase only supports specific operators: ==, <, >, <=, and >=. Using any other operator causes a syntax error.

3. Final Answer:

   '=>' is invalid operator causing error -> Option C

4. Quick Check:

   Use >=, not => for greater or equal [OK]

Hint: Use >=, not => for greater or equal [OK]

Common Mistakes:

• Typing => instead of >=
• Using strings instead of numbers for numeric fields
• Assuming all operators are valid

5. You want to query Firebase documents where the field temperature is between 20 and 30 inclusive. Which query correctly uses comparison operators to achieve this?

hard

A. query.where('temperature', '>', 20).where('temperature', '<', 30)

B. query.where('temperature', '>=', 20).where('temperature', '<=', 30)

C. query.where('temperature', '==', 20).where('temperature', '==', 30)

D. query.where('temperature', '<=', 20).where('temperature', '>=', 30)

Solution

1. Step 1: Understand inclusive range requirements

   Inclusive means including both 20 and 30 values.

2. Step 2: Choose operators that include boundary values

   >= includes 20 and <= includes 30, so these operators match the requirement.

3. Step 3: Verify the combined query

   The query query.where('temperature', '>=', 20).where('temperature', '<=', 30) correctly filters temperatures from 20 to 30 inclusive.

4. Final Answer:

   query.where('temperature', '>=', 20).where('temperature', '<=', 30) -> Option B

5. Quick Check:

   Inclusive range uses >= and <= [OK]

Hint: Use >= and <= for inclusive ranges [OK]

Common Mistakes:

• Using > and < excludes boundary values
• Using == twice filters only exact matches
• Reversing operators causing empty results